(1) Field of the Invention
This invention relates to an electrical insulating oil and oil-filled electrical appliances which is impregnated with the same.
More particularly, the invention relates to an electrical insulating oil and oil-filled electrical appliances impregnated with the same where the electrical insulating oil comprises a heavier fraction which is prepared by disproportionating diarylalkane or a hydrocarbon mixture mainly containing diarylalkanes.
(2) Description of the Prior Art
Electrical appliances such as oil-filled capacitors and oil-filled power cables have recently been made small in size and light in weight. With this tendency, at least a part of the electrical insulating materials or dielectric materials is made of plastic materials like polyolefins such as polypropylene.
In the oil-filled electrical appliances, several improved measures are taken with regard to the structures of the electrical appliances, however, there have never been proposed any satisfactory electrical insulating oil to be used for impregnation. That is, the conventional electrical insulating oils such as refined mineral oils, polybutenes, alkylbenzenes, diarylalkanes, alkylbiphenyls and alkylnaphthalenes are not always satisfactory in their electrical characteristics. Among several kinds of oil-filled electrical appliances, especially, in metallized film capacitors (hereinafter referred to as "MF capacitors") in which a metal-deposited film that is prepared by depositing in vacuum a metal such as aluminum or zinc on a plastic film, is wound as an electrode and it is impregnated with an electrical insulating oil, there is scarcely proposed any electrical insulating oil suitable for impregnation.
In other words, presently used MF capacitors are mainly the so-called dry type MF capacitors in which an electrically insulating impregnation substance such as electrical insulating oil is not used. Not only in electric capacitors but also in general electrical appliances, the potential gradient can be made high by surrounding electrodes or electric conductors with an impregnating agent of electrical insulator. Accordingly, the voltage-withstanding level of the so-called impregnation-type MF capacitors can be made higher than that of dry-type ones, and it becomes possible to comply with the requirement to make them light and small. However, the metallized films having a base film of plastic such as polypropylene film, are liable to be influenced by impregnating oils. For example, when change in the size of base film due to the impregnation of an impregnating oil or permeation of the impregnating oil into the boundary between a deposited metallic layer and a base film, is caused to occur, cracks are formed in the deposited metallic layer and, what is worse in many cases, the metallic layer is peeled off to cause dielectric breakdown. Therefore, there are few electrical insulating oils which are suitable for use in MF capacitors.
Meanwhile, it is widely put into practice in the industry to produce ethylbenzene, ethyltoluene or cumene by alkylating benzene or toluene with ethylene or propylene in the presence of an alkylation catalyst. The ethylbenzene and ethyltoluene are dehydrogenated into styrene and vinyltoluene which are used as the monomers for producing styrene-type polymers. Cumene is used as a starting material for cumene-phenol process.
From the above alkylation process, a by-product oil fraction containing 1,1-diarylalkanes is obtained. It was proposed to use the fraction as an electrical insulating oil (U.S. Pat. No. 4,111,824).
The fraction proposed in the above reference, however, is not always satisfactory for use in oil-filled capacitors, especially, in MF capacitors even though it is available in large quantities at low cost.
Furthermore, it is proposed to use aromatic hydrocarbons such as distyrenated xylene as an electrical insulating oil which are heavier than diarylalkanes. However, the pour points and viscosities of heavier aromatic hydrocarbons are usually high and thus few of them are used practically.